1. Field of the Invention
The present invention relates to communication systems. More specifically, the present invention relates to a novel technique for efficient processing of a large number of trials in time and frequency to "find" a transmitted code or to decode a frequency coded signal. The invention is applicable to radar processing to generate a range/doppler map efficiently from an arbitrary coded signal.
2. Description of the Related Art
A number of multiple access signal transmission schemes have been defined, including TDMA (Time Division Multiple Access,) FDMA (Frequency Division Multiple Access,) and CDMA (Code Division Multiple Access). In general terms, TDMA involves very high speed packet modulation of a single waveform in a channel, where a channel is defined by a slice of time as measured by a defined offset from a synchronization (sync) pulse. TDMA transmission thus splits time for transmission into different segments, and sends different pieces of information in respective segments. Dividing a transmission according to time in this way requires very high speed analog-to-digital (A/D) converters, and very precise time control.
Rather than dividing transmission time into segments, FDMA splits a frequency band into channels which are sub-bands, and sends signals simultaneously across the various channels. As a result, transmitted signals tend to be relatively narrowband, and thus more susceptible to multipath distortion. Also, if the original transmission band is sufficiently large, it is possible that some of the sub-bands will traverse harmonics of other sub-bands at lower frequencies. This traversal can cause spurs which could wipe out specific channels. In addition, a linear operation is required to reduce the "mixing" of these channels. Such a relationship can force a decrease in available power in order to retain the requisite linearity.
CDMA uses simultaneous coded channels over a wide band. While this approach eliminates most of the problems associated with TDMA and FDMA, CDMA requires accurate time and frequency control to avoid time of arrival shift problems and doppler problems.
Given the various limitations of the foregoing approaches, it would be desirable to provide a transmission scheme which did not require the level of accuracy to prevent distortion, and which retains the advantages of wideband transmission.